Attaching electronic devices to communication channels has always been fraught with peril. Specifically, there is an inherent danger to the integrity of any electronic circuit whenever there is a possibility that portions of the circuit can be exposed to large voltage potentials. One example of this situation can be seen in modulator/demodulator circuits; commonly called “modems”.
In order to lessen the probability of such voltage induced damage, modems and telephone systems, in general, have long utilized isolative circuits. Isolative circuits introduce an electrical, or “galvanic” barrier between a first electronic system and another. Isolative circuits are also known to protect electronic circuits from unintended voltage potentials induced by static electricity, e.g. lightening strikes. In modem applications, isolative circuits are normally introduced between the actual modem function and a communications channel that is used to convey a modem signal to a remote location.
An isolative circuit generally comprises a power transformer that is used to generate an isolated power source. This isolated power source can be used to power an isolated portion of a circuit. In a modem circuit, the isolative circuit further comprises a collection of high-voltage capacitors. These capacitors straddle the galvanic barrier and are used to convey information between a non-isolated portion of the circuit and an isolated portion of the circuit. Because modems are typically used to communicate with a telephone-line based communications channel, the non-isolated portion of the circuit is generally referred to as the “system side” whereas the isolated portion of the circuit is referred to as the “line side”.
Up until now, the conveyance of data across the galvanic barrier has been accomplished by exploiting the alternating current (AC) nature of the data signals traveling outward from the system side and inward from the line side. The term “AC” is used here in reference to any signal that varies in voltage over time. Because these data signals exhibit AC characteristics, they are able to “jump” the galvanic barrier just like any other AC signal; they are able to flow through a capacitor. Direct current (DC) components in the signals, such as any unwanted high-voltage level, are effectively blocked by the capacitors. Hence the old axiom-capacitors can be thought of as an open circuit with respect to DC signals.
Everyone knows that price competition drives every aspect of the economy. The personal computer market is no exception and continues to see falling prices as competition continues to rage. Any modem that is designed for mass production must be sensitive to this. One problem that has continued to plague modem manufacturers is the cost associated with the isolative circuit. The isolative circuit simply cannot be abandoned because the galvanic barrier that it provides is necessary not only to protect the system from inadvertent exposure to high voltage, but also to comply with various design requirements that govern communications equipment. For example, these design requirements dictate that an isolative barrier capable of withstanding 4,000 Volts of potential difference must be provided in a modem.
Examining the components necessary to implement the galvanic barrier, it quickly becomes apparent that the transformer and the high-voltage capacitors represent a significant portion of the material cost of a typical commercial modem. It would be advantageous if these costs could be either reduced or eliminated entirely.